Implements the proposed design (with some modifications) in #135058.
## Sequencer Scene
This adds a new property called `sequencer_scene` to workspaces. This scene is used
by the video sequence editors in the current workspace for their context.
This is a first step towards "detaching" the VSE from the active scene in the window.
Each sequencer timeline editor shows the sequencer scene that is being used.
By default, when no sequencer scene is selected, the timeline and preview are empty.
Pressing the "new" button will add a new scene and assign it to the sequencer
scene for the current workspace.
## Contextual Playback
Pressing `Space` (by default) for starting the animation playback is now contextual:
depending on the context (where your mouse cursor is), the scene that is played back
might be different. E.g. with a 3D Viewport and a Sequencer open, pressing "play"
in the 3D Viewport will play the _active scene_ of the window, while pressing "play"
in the sequencer will play the _sequencer scene_.
## Time & Scene Synchronization
Additionally, this adds a toggle called "Sync Active Scene".
With the property turned on, the active scene & scene time in the window will be
synced with the time & scene of the current scene strip in the sequencer.
Note that this is _not_ bi-directional. The sequencer can change the active scene
and map time, but it's not possible the other way around since it one can have
multiple strips using the same scene (+camera, and even time!).
Currently this setting is exposed in the footer of the sequencer timeline as well
as in the workspace settings.
This allows for one of the core concepts that the story tools projects aims at: Working
in a scene (e.g. in the 3D viewport) while also working with the edit
(in the sequencer timeline).
## Some technical notes
* Undoing while playback is running will now cancel playback. This is to avoid the timer,
that points to the scene and viewlayer that are playing, to get de-synced after loading
the memfile undo step.
* When the sequencer scene is not the same as the active scene, we ensure it has
a depsgraph.
* Normally, when a `NC_SCENE` notifier points to a specific scene, the notifier is dropped
if that scene doesn't match the active one in the window. We now also check that it
doesn't match the sequencer scene in the active workspace.
* When loading older files, we need to make sure that the active workspace in a window
uses the active scene as the sequencer scene. This is to make sure that the file opens with
the same sequences open.
* Tool settings are stored per scene. To make sure the sequencer uses the tool settings for
the sequencer scene, the "context.tool_settings" and `CTX_data_tool_settings` members
are overridden in the sequence editors.
Pull Request: https://projects.blender.org/blender/blender/pulls/140271
1122a05cb6